An anomaly handling method that suitably handles a case where the possibility of a vehicle being unauthorizedly controlled so as to suppress the effects thereof is provided. In an anomaly handling method used in one or a plurality of electronic control units installed in one vehicle, an inter-vehicle communication message transmitted from a device installed in the other vehicle is received as an anomaly detection notification, the anomaly detection notification being issued when an unauthorized frame is detected on an onboard network installed in another vehicle, and an anomaly handling processing is selected from a plurality of predetermined anomaly handling processing in accordance with the received content to transition to a safe state for example, and the selected anomaly handling processing is executed.